Treatment of oxide coatings on aluminum



Patented Feb. 7, 1950 TREATMENT OF oxina COATINGS N ALUMINUM Walter G. Zelley, NewKensington, Pa., asslgnor to Aluminum Company of America, Pittsburgh, -Pa., a corporation of Pennsylvania No Drawing. Application May 6, 1947, Serial No. 746,307

18 Claims. 1

This application is a continuation-in-part of my prior application, Serial No. 716,431, now abandoned, entitled Treatment of oxide coatings on aluminum.

This invention relates to the treatment of oxide coatings on aluminum so as to render them more adsorptive, especially with respect to dyes. The invention is particularly concerned with a new coating-modifying treatment that may be interposed between the usual coating-forming treatment and various other modifying treatments for oxide coatings on aluminum.

As used herein, the word aluminum includes pure aluminum, commercial aluminum. containing the usual impurities and aluminum base alloys. All are susceptible of being oxide-coated and treated by the methods that are discussed herein.

The art of oxide coating aluminum articles is well advanced, but commercial applications of the art give rise to some difiiculties in preparing uniform coatings and in effecting'uniform adsorption of coloring or other materials therein.

Two general types of artificially produced oxide coatings on aluminum are commonly recognized. One type is formed by electrolytic treatment, usually in acid electrolytes, the aluminum article being made anode in the electrolyte and having an oxide coating formed on its surface by electrolytic oxidation of the surface metal. The most popular coating of this type is produced in a sulfuric acid electrolyte. The other type is formed by chemical treatment, without the application of external electrical energy, usually in alkaline solutions, the aluminum article being merely immersed in the solution and having an oxide coating formed on its surface by oxidation of the surface metal. The most popular coating of this type is produced in an alkaline carbonate type solution, usually a sodium carbonate-chromate solution.

Electrolytically produced oxide coatings generally are adsorptive and easily dyed or otherwise colored, but they are relatively expensive compared to chemically produced oxide coatings. the other hand, oxide coatings chemically produced on aluminum articles in alkaline solutions generally are less adsorptive or less easily dyed, although they are relatively inexpensive.

It is therefore seen that the invention has application only to the second type of coating. the type which is chemically produced on an aluminum article in an alkaline solution. Consequently, the language oxide coating chemically produced on an aluminum article in an alkaline I) v a. solution" is used in the specification, and may also be used in the claims, to refer to an artificially produced oxide coating obtained by chemical treatment in an alkaline solution without the application of external electrical energy, but it is not intended to include the thin natural film of oxide occurring on any aluminum surface which has been exposed to the atmosphere.

General objects of the invention are to render certain oxide coatings on aluminum more adsorptive, especially with respect to dyes. Other objects include simplifying the controls that must be exercised over the usual coating-forming and modifying treatments for certain oxide coatings on aluminum by providing a treatment which will modify the coating so as to make it more receptive to various additional treatments. Further objects and advantages of the invention will appear hereinafter.

I have found that I can so treat an oxide coating that has been chemically produced on an aluminum article in an alkaline solution as to render it more adsorptive, especially to dyes, by immersing thecoated article in an aqueous solution containing at least one of several suitable ammonium compounds, preferably a hot solution. It is believed that this treatment opens pores in the coating, but possibly it modifies the coating in some other way. A suitable ammonium compound may be selected from the group consisting of ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium sulfamate, which group may be described as being made up of ammonium salts of monovalent acids (inorganic, organic or mixed). I have found, however, that ammonium fluoride, which is also a salt of an inorganic monovalent acid, is not desirable for the practice of the invention, since it exhibits active solvent properties with respect to aluminum oxide coatings. A typical procedure for application of the invention is given below, with indications as to variations that can be made in the procedure.

The coated aluminum article is merely immersed for about 1 to 20 minutes, preferably about 5 to 10 minutes, but for a longer time if desired, in a solution of one of the above-named ammonium compounds. The solution is best maintained at temperatures above about F., and is preferably kept near boiling, but longer immersions in solutions of high concentrations are effective at temperatures as low as room temperature. The ammonium compound concentration may be about 0.5 to 10 per cent by weight,

or even up to the saturation limit of the compound in water, but preferably is about 1 per cent. The pH of the solution will fall between about 5 and 7.5, and the solution is preferably employed at a pH between about 5.3 and 7. If the pH of the solution is lowered below about 5 the treatment tends to make the coating chalky;. and if the pH is raised above about 7.5, the solution loses.

effectiveness.

Following the coating treatment and before;

the step just described, the oxide coated aluminum article is preferably rinsed in cold water.,

Likewise, it is preferably rinsed in water after treatment with the ammonium compound; Best results are obtained if it is then immediately substantial amounts, such as 568 alloy which contains about 5.25 per cent magnesium, or 528 alloy which contains about 2.5 per cent magnesium. Further, the treatment withammonium acetate, for example, has particularly beneficial application to oxide-coatings on aluminum base alloys containing magnesium and copper, or magnesium, copper and zinc, such as 245 alloy which contains about 1.5 per cent magnesium and about 4.5 per cent copper, or 75S alloy which contains about 2.5 per cent magnesium, about 1.6 per cent copper and about 5.6 per cent zinc. The invention may be practiced with coatings on other aluminum base alloys and aluminum' of varying degrees of purity, of course, but is not quite as efiective with coatings on commercially pure aluminum, such as 2S, or with-coatings on other magnesium-free alloys of aluminum; With respect to coatings on magnesium-free aluminum, the treatment has a tendency to make the coating soft and chalky.

As pointed out above, the invention is particularly applicable to the treatment ormodifi cation of oxide coatings formed by chemical treatment in alkaline solutions. Of the general class of such oxide coating treatments for aluminum. the treatment in an alkaline solution ofthe carbonate type has become the most sige nificant commercially, and the invention is particularly applicable thereto.

A typical carbonatetype of solution comprises about 2 per cent by weight of an alkali metal carbonate and about 0.1 per cent by weight of alkali metal chromate or dichromate. Thecarbonate usually is sodium carbonate, and its concentration may range from about 0.5 to about 6 percent. The chromate is usually sodium chromate or sodium or potassium dichromate, and its concentrationmay range from about 0.05 to about 1 per cent, although under some conditions the chromate content may not be essential.

The carbonate type of solution may be used with additional components such as polyvinyl alcohol or a protein (albumen, zein, casein or gelatin) present in amounts by weight'between about 05 and 5 per cent, but usually about 1 per cent. Especially with such additional components, but not necessarily only in such cases, the chromate content of the solution may be greatly reduced or eliminated.

The designation carbonate type for an oxide coating solution for aluminum includes all of the: above described solutions, while the designations carbonate-chromate, carbonate-polyvinyl alcohol and carbonate-protein include solutions containing at least the components named. These and other short names for the solutions to which the invention is especially applicable may be used in the claims with the meaning indicated.

What is claimed is:

l. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a solution consisting essentially of water and at least one ammonium compound. from the group ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium sulfamate as a coating modifying component.

2; The method of" rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comsisting: essentially of water and about 0.5'to 10 per cent by weight of an ammonium compound:

from the group ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium sulfamate as the coating modifying component, for a period of about 1 to 20 minutes.

4. The method of rendering an oxide coating chemically-produced" on an aluminum article in an alkaline solution more adsorptive which comprises-immersing said article in a solution'cona sisting essentially of water and about 05 to 10 per cent by weight of an ammonium compound from thexgroup ammonium chloride, ammonium nitrate; ammonium formate, ammonium acetate and ammonium sulfamate as the coating modifyingcomponent and maintained at a temperature above about F.

5. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises. immersing said: article ina solution consisting essentially of water and an ammonium compound from the group ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium sulfa-mate as the coating modifying component and maintained at atemperature above about 170 F., for a period-ofaboutl to-20 minutes.

6. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article inv a solution consistingessentially of water and about 0.5 to 10 percent by weight of an ammonium compound from the group ammonium.- chloride, ammonium nitrate, ammonium formate, ammonium acetate and" ammonium sulfamateas the coating modifying component and; maintained at a temper-..

ature above about 170 F., for a period of about 1 to 20 minutes.

7. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive to dyes which comprises immersing said article in a solution consisting essentially of water and about 1 per cent by Weight of an ammonium compound from the group ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium sulfamate as the coating modifying component and maintained at a temperature above about 170 F. and a pH of about 5 to 7.5, for at least about 5 to minutes.

8. In a process of coating and coloring an aluminum article, wherein the article is provided with an oxide coating in an alkaline carbonate type of chemical coating solution and then colored in a dyeing solution, the improvement which comprises rendering the coating produced in the first solution more adsorptive to dye in the second solution by first rinsing the coated article in water, then immersing said coated article in a hot solution composed of water and about 1 per cent of an ammonium compound from the group ammonium chloride, ammonium nitrate, ammonium formate, ammonium acetate and ammonium suliamate for about 5 to 10 minutes, and then again rinsing said coated article preparatory to coloring.

9. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a solution consisting essentially of water and ammonium chloride as the coating modifying component.

10. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a hot solution composed of water and ammonium chloride.

11. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a solution consisting essentially of water and ammonium nitrate as the coating modifying component.

12. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a hot solution composed of water and ammonium nitrate.

13. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a solution consisting essentially of water and ammonium acetate as the coating modifying component.

14. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a hot solution composed of Water and ammonium acetate.

15. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive Which comprises immersing said article in a Solution consisting essentially of water and ammonium formate as the essential coating modifying component.

16. The method of rendering an oxide coating chemically produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a hot solution composed of water and ammonium formate.

17. The method of rendering an oxide coating chemicall produced on an aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a solution consisting essentially of water and ammonium sulfamate as the essential coating modifying component.

18. The method of rendering an oxide coating .chemically produced on an .aluminum article in an alkaline solution more adsorptive which comprises immersing said article in a hot solution composed of water and ammonium sulfamate. WALTER G. ZELLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS- 

7. THE METHOD OF RENDERING AN OXIE COATING CHEMICALLY PRODUCED ON AN ALUMINUM ARTICLE IN AN ALKALINE SOLUTION MORE ADSORPTIVE TO DYES WHICH COMPRISES IMMERSING SAID ARTICLE IN A SOLUTION CONSISTING ESSENTIALLY OF WATER AND ABOUT 1 PER CENT BY WEIGHT OF AN AMMONIUM COMPOUND FROM THE GROUP AMMONIUM CHLORIDE, AMMONIUM NITRATE, AMMONIUM FORNATE, AMMONIUM ACETATE AND AMMONIUM SULFAMATE AS THE COATING MODIFYING COMPONENT AND MAINTAINED AT A TEMPERATURE ABOVE ABOUT 170*F. AND A PH OF ABOUT 5 TO 7.5, FOR AT LEAST ABOUT 5 TO 10 MINUTES. 